In recent years, there have frequently occurred incidents of information leakage when a portable storage medium (portable medium) such as a USB (universal serial bus) memory or a CD (compact disc) where a file having business secret or personal information is copied is lost or stolen after being taken out. Occurrence of such incidents is a social problem.
In order to prevent such information leakage, Patent literature 1 discloses a technology for automatically encrypting and storing a file in a portable medium for making a copy of the file in the medium. That is, by having the file thus encrypted and stored, a person who does not know a decryption key is prohibited from seeing the stored information even in a case where loss or the like of the portable medium occurs. Thereby, a risk of information leakage can be reduced.
Additionally, in order to prevent information leakage from e-mails, Patent literature 2 discloses a technology with which: an e-mail transmitted by a client PC (personal computer) or the like is received on a network; an encrypted e-mail is created by encrypting a text body and an attached file of the received e-mail; and the encrypted e-mail is transmitted to a transmission destination of the original e-mail. That is, even in the case where the e-mail is intercepted on the network, the above encryption makes it impossible for a person who does not know a decryption key to see information stored in the e-mail. Thereby, a risk of information leakage can be reduced.
Furthermore, a so-called mail filtering technology is well known. The mail filtering technology prohibits transmission of an e-mail by determining, for example, whether or not a text body or an attached file of the e-mail contains any predetermined term (for example, “strictly confidential”, or “personal information”), from which a risk of information leakage is predicted.